Fiber-reinforced ceramic matrix composites comprising glass-ceramic matrices are known in the art. Fiber-reinforced ceramic matrix composites are useful as structural elements in high temperature environments such as heat engines. For these and other applications, the materials to be employed must exhibit good strength and toughness at ambient as well as elevated temperatures.
An important problem which has been identified in silicon carbide fiber reinforced ceramic matrix composites, particularly after exposure to temperatures above about 800.degree. C. in an oxidizing environment, is that microcracks can form causing embrittlement. Instead of exhibiting increased toughness and strength after exposure to high temperatures, the materials become brittle and are subject to catastrophic breakage, rather than more gradual failure as is typical of the original material. These physical problems can be attributed, in-part, to the effect of the interface between the silicon carbide fibers and the ceramic matrix composite.
Physical testing of ceramic matrix composites, embrittled during or subsequent to high temperature exposure, shows decreases in fracture toughness through changes in the fracture properties of the material, leading to a degradation of the material. Thus, the predominant fracture mode changes from one characterized by fiber pullout from the matrix to one wherein woody fracture, or ultimately, brittle fracture occurs. Woody fracture surfaces display some crack propagation parallel to the stress axis, indicating localized shear failure without fibrous pullout, while brittle fracture surfaces display merely planar fracture surfaces as the composite exhibits no plastic deformation.
The onset of brittle fracture behavior in these composites typically occurs in conjunction with significant reductions in fracture toughness. One indicator of this reduced toughness is a drop in the extent of strain of sample elongation observed above the so-called microcrack stress point of the material. Among the factors believed to influence fracture toughness are fiber debonding and fiber pullout behavior, including the degree of frictional resistance to fiber pullout from the matrix, as well as crack deflection occurring in the matrix and at the fiber-matrix interface. Modifications to the matrix or fiber reinforcement can significantly aid in the development of composites exhibiting good high temperature fracture toughness and strength.
It is known to provide coatings on reinforcement fibers to be incorporated in composite materials to modify the behavior of the materials therein. For example, boron nitride coatings have been applied to silicon carbide fibers or other fibers that are subsequently incorporated in ceramic matrix materials such as SiO.sub.2, ZrO.sub.2, mullite and cordierite (see e.g., U.S. Pat. No. 4,642,271 (Rice)).
It is established that the interface between fibers and the matrix is critical to the mechanical properties of brittle-matrix composites. In particular, the debonding and frictional characteristics of the interface control the mode of fracture (multiple cracking vs. single crack), and mechanical properties such as fracture toughness. Desired interfacial properties are usually achieved by the incorporation of a coating between the fiber and matrix.
For example, Beall et al., European Patent Application Publication Number 366234 A1, disclose ceramic matrix composite articles comprising a ceramic, glass-ceramic or glass matrix and a fiber reinforcement phase disposed within the matrix. The fiber reinforcement phase consists of amorphous or crystalline inorganic fibers, wherein there is provided, on or in close proximity to the surfaces of the inorganic fibers, a layer of sheet silicate crystals. The layer of sheet silicate crystals are used to improve fiber pullout behavior and to improve toughness retention at elevated temperatures.
At present, however, there are only a few other successful coating materials, most notably, carbon, although some success has been reported with metallic and porous coatings. In most of the composite systems that have been studied to date, exposure of the coating to high temperatures in air seriously degrades its properties. For example, in the case of lithium aluminum silicate matrix reinforced with carbon-coated silicon carbide fibers, heat treatment in air leads to a strong SiO.sub.2 interface, and the material loses its quasi-brittle mechanical properties. There is therefore great interest in developing alternative coatings for fibers in brittle-matrix composites.
Oxides are a class of materials which have intrinsic high temperature stability in air. A particular interest has been to look at using oxidation resistant materials as potential fiber coatings in ceramic matrix composites for high temperature/high stress applications. An important consideration in choosing an interfacial material is its ability to form uniform coatings on the fibers in question.
Clearly, coating materials having excellent film-forming capability, and which can be coated successfully onto fibers such as SiC and borosilicate glass fibers, are needed. Such materials need to provide debonding coatings on the surfaces of fibers used in ceramic matrix composites, wherein such coatings remain stable at elevated temperatures. Moreover, such ceramic matrix composites need to have high strength and fracture toughness, even at elevated temperatures. As a result, it is an object of the present invention to provide coatings for fibers and ceramic matrix composites that overcome the problems and deficiencies of the prior art. Other objects and advantages of the present invention will become apparent to those skilled in the art upon reference to the attached drawings and to the detailed description of the invention which hereinafter follows.